During the 2011–12 season Colgan began coaching youngsters back at former club Barnsley as well as continuing to play with Huddersfield. In July 2012 he signed a new contract with Huddersfield to make him the club's academy goalkeeping coach as well as still making himself available as a player.

Born in Drogheda, County Louth, Colgan began his football career with his hometown club Drogheda United in 1991. He remained at United Park for one season as a professional while he was scouted by overseas clubs. This would eventually see Colgan depart United and move to England.

Colgan signed professional terms with Chelsea, who at the time were a founding club of the newly designed FA Premier League in the summer of 1992, although he had appeared in their youth team 6 times in 1990/91, becoming their regular goalkeeper for the following season. The Blues were managed at the time by Ian Porterfield and Colgan was designated as the third choice keeper at Stamford Bridge behind Dmitri Kharine and Kevin Hitchcock. In February 1993, Porterfield was sacked and was replaced by player-manager Glenn Hoddle, who in turn put Colgan up for loan. In September 1993, Colgan was signed by Dario Gradi in a one-month loan spell with Crewe Alexandra, however he failed to make an appearance for the club and soon returned to Stamford Bridge. During the 1994–1995 season joined up with Alan Buckley at Grimsby Town who used him as a deputy keeper to Paul Crichton, but also Colgan went unused at Blundell Park during his spell in Humberside. During the 1995–1996 campaign, Colgan joined London city rivals Millwall on a further loan deal but he again failed to make a first team appearance. For the 1996–1997 season, Hoddle was replaced by Ruud Gullit who handed Colgan his first team professional debut playing in a 3–2 away defeat at the hands of West Ham United on 12 March 1997. Colgan had filled in for the then current first choice Frode Grodas who sat the game out on the bench. The Chelsea team that day featured the likes of Gianfranco Zola, Gianluca Vialli, Dan Petrescu, Dennis Wise and Mark Hughes. This would be his only taste of first team football for Chelsea, however he would go on to make 5 loan appearances each for both Brentford and Reading during the 1997–1998 season. In the summer of 1998, Chelsea now managed by Gianluca Vialli released Colgan from the club.

In July 1998, fresh from his release from Chelsea, Colgan signed on a free transfer for Bournemouth. Colgan would make not one single appearance for the club during his only season at Dean Court, with him playing second fiddle to Mark Ovendale for the entire season.

During the summer of 1999, Colgan signed with Scottish Premier League club Hibernian. His first season at Easter Road he made 31 appearances for the club, and was looked upon as the first choice keeper in a side that boasted Olafur Gottskalksson and John Campbell. He went on to make a total of 147 appearances in all competitions between joining in 1999 and his departure in the summer of 2004. During the 2003–2004 season, Colgan was loaned out, where he spent most of the season with Stockport County. County were under a squad re-shuffle that had seen them use Lee Jones, James Spencer, Boaz Myhill, Anthony Williams and Colgan in the net that season. At the end of the season, Colgan was linked with a permanent move to Stockport but the transfer never materialised.

In the summer of 2004, Colgan signed for Barnsley, where he endured a tough first season only playing in 15 games and ending up on loan to Dundee United for 3 months. The 2005–2006 Barnsley campaign showed a turn of fortune for Colgan as he became the team's first choice keeper. His outstanding displays during the season earned him 20 clean sheets in total as well as the title of Barnsley's Player of the Year and helped the team reach the League One Play-off final at the Millennium Stadium in Cardiff on 27 May 2006 where despite a mistake letting Swansea City take the lead, he recovered to make a historic save from Alan Tate in the penalty shoot out which ensured Barnsley won their place back in the Championship.On 4 October 2007 he handed in a transfer request at Barnsley after losing his place to summer arrival Heinz Müller. Manager Simon Davey accepted the request.[3]

He signed for Ipswich Town on 31 January 2008 as a free agent after settling his contract at Barnsley until the end of the season. He failed to make an appearance for Town, after fellow keeper Stephen Bywater was loaned in from Derby County who kept the goalkeeping shirt for the remainder of the season. Colgan was not offered a new contract at the end of the season and instead started the 2008–09 pre-season by training with Bradford City in the hope of earning a contract with the League Two club.[4] He played his first game in more than 8 months on 16 July 2008 when Bradford City played Farsley Celtic in a pre-season friendly.

On 24 July 2008, Colgan returned to the Premier League and signed with Sunderland under the tenure of Roy Keane. The idea of Colgan's move was to provide cover for first choice keeper Craig Gordon, as well as competing for a place on the bench with fellow back-up keepers Darren Ward and Marton Fulop. Colgan was named as a substitute on several occasions, but failed to make an appearance for The Black Cats.[5] He was eventually released on 28 May 2009 by Keane's replacement Ricky Sbragia along with six other first team players, notably Dwight Yorke, Rade Prica and David Connolly.[6]

Colgan dropped three leagues down and re-signed for his former club Grimsby Town on 13 July 2009. He had previously spent a month on loan with Grimsby 15 years earlier whilst the club were a mid-table team in what is now the Championship, but Grimsby side had since dropped two divisions to League Two. Colgan signed for The Mariners after being recommended to manager Mike Newell by his former boss at Sunderland, Roy Keane.[7] Colgan stepped into a squad with only teenagers as his cover, Leigh Overton and Ed Eley, neither of whom had any competitive outings in the first team.

Colgan's second stint at Blundell Park got off to a slow start as an early season injury put him out of action for 6 weeks. Mike Newell replaced Colgan with loan signings Tommy Forecast and Josh Lillis. Colgan made his return from injury on 30 September in a 3–2 defeat against Chesterfield.

Over the Easter weekend period, injuries to both Colgan and his young understudy Overton meant that Mark Oxley was signed on an emergency loan to deputise. Colgan returned to the side for the away victory against Hereford United on 10 April. In what was his final game for the club he was assaulted by one of his own fans when a fan ran onto the pitch to confront him moments after he had conceded the first goal in a 3–0 defeat at Burton Albion that condemned Grimsby to relegation.[8] Colgan activated a clause in his contract which meant he was able to leave the club in the event of relegation. Manager Neil Woods commented that it was up to Colgan if he wished to leave the club. It was also stated that the signing of Rochdale's Kenny Arthur as Grimsby's new first choice keeper could sway Colgan's decision into leaving the club in favour of remaining as Town's back up goalkeeper for the upcoming season.[9]

After failing to agree terms with another club during pre-season, Colgan returned to Grimsby for pre-season training. On 15 October 2010, Colgan moved on a month's loan to Football League One side Huddersfield Town, as cover for the injured Alex Smithies and the experienced Ian Bennett. He made no appearances and returned after one month. It was reported on 17 November 2010 it was stated by Grimsby chairman John Fenty that Colgan had no future at the club[10] and on 7 January 2011 Colgan was released from his contract by mutual consent.[11]

On 21 January 2011, Colgan re-signed for Huddersfield Town on a contract running to the end of the season, mainly as back-up for Ian Bennett, while Alex Smithies is recovering from injury. He finished the 2010–11 season without playing a single league game for The Terriers, but made his debut in a 3–0 victory over Carlisle United in the Football League Trophy. During the 2011–12 season, Colgan remained the club's third choice goalkeeper, making two appearances all season, one in the FA Cup and another in the Football League Trophy. Despite not featuring as a regular in the league he was named as a substitute in the Football League One play-off final victory over Sheffield United at Wembley Stadium. On 29 May 2012 it was announced that Colgan would be appointed as full-time academy goalkeeping coach at Huddersfield for the 2012–13 campaign, but the club would retain his playing registration.[12]

Colgan made his Republic of Ireland début in March 2002, against Denmark. He went on to make a total of ten appearances for his country,[2] but was selected many more times for the national squad as a backup to the likes of Shay Given and Paddy Kenny.

Colgan worked part-time as a goalkeeping coach with Barnsley's academy while playing for Huddersfield Town.[12] At the end of the 2011–12 season, Colgan was appointed to a similar full-time role at Huddersfield Town.[12]

In July 2014 Colgan made the step up to the club's first team goalkeeping coach, with Ian Bennett replacing him in his previous role with the academy.[13] In June 2017, following Huddersfield's promotion to the Premier League, Colgan was sacked as the club's goalkeeping coach. He was reported to have consulted the Professional Football Coaches Association about possible legal action for wrongful dismissal.[14] In July 2017 he was appointed to the coaching staff at Wigan Athletic.[15]